The big picture: The environment influences populations




  • Review: What is a population?


  • Biotic and abiotic factors affect population dynamics
    • abundance, dispersion & age structure


  • Population growth can be ‘modeled’
    • tracking births & deaths can show trends


  • Density (#/area) matters
    • crowdedness impacts population growth

Populations often described by their boundaries

Measuring population size and density



  • May be possible to count all individuals
    • large mammals, animals in small habitats


  • More often impractical or impossible to count all individuals
    • indirect estimates are needed
    • dusk deer surveys in Biol 303


  • Population size and density is dynamic
    • migration in and out of populations
    • births & deaths
    • motility

Densities vary within a population

The density of these populations is the same but their ecology differs

Tracking population trends through time (demography)


  • Reproductive rates vs Survivorship
    • key for conservation


  • Cohorts: group of individuals born at the same time
    • how many of one age-group survive to the next age level?
    • when does reproduction occur?


  • Female power!
    • only females produce offspring
    • how many females give rise to new females`?
    • how many years do females reproduce?

Trade offs exists between reproduction and survival




  • Frequency of reproduction
  • Number of offspring
  • Investment into parental care


Combined, the traits represents an organism’s life history

  1. Age at first reproduction?
  2. How often reproduction occurs?
  3. How many offspring are created?
  4. What happens to offspring after birth?

Trade offs exists between reproduction and survival




  • Frequency of reproduction
  • Number of offspring
  • Investment into parental care


Combined, the traits represents an organism’s life history

  1. Age at first reproduction?
  2. How often reproduction occurs?
  3. How many offspring are created?
  4. What happens to offspring after birth?

Trade offs exists between reproduction and survival




  • Frequency of reproduction
  • Number of offspring
  • Investment into parental care


Combined, the traits represents an organism’s life history

  1. Age at first reproduction?
  2. How often reproduction occurs?
  3. How many offspring are created?
  4. What happens to offspring after birth?

Patterns of survivorship in natural populations

Populations grow fast when resources are abundant

Can we predict changes in population size?


  • Start with ideal conditions:
    • few individuals to start
    • non-limiting environment


  • Population ↑ with births & immigration


  • Population ↓ with deaths & emigration


  • Balance of births & deaths at any given time
    • per capita change (r) =

Elephants in Kruger National Park (Exponential Growth)

Resources rarely remain non-limiting



  • Carrying capacity (K):
    • as population increases, resources diminish


  • Resources that limit are vast
    • food, shelter, safety, nesting sites


  • Resource limitation causes ‘r’ to decline


  • Logistic growth =

Density in populations matter (carrying capacity)



  • Why do populations stop growing?


  • Density dependent factors = crowdedness
    • resources, predator-prey
    • disease, waste becomes toxic


  • Population density works as a negative feedback for predator-prey populations
    • creates a cyclical relationship where both populations are regulated by the other’s density.

What factors are density independent?

Patterns in human population

Patterns in human population: Age structure

Patterns in human population: Density and carrying capacity